Houston — Mattress Firm Holding Corp., the nation’s largest specialty mattress retailer, has entered into an agreement to acquire all of the outstanding equity interests in HMK Mattress Holdings LLC, the holding company of Sleepy’s, for $780 million. Sleepy’s is the nation’s second largest specialty mattress retailer, with more than 1,050 stores in 17 states in the Northeast, New England the Mid-Atlantic and Illinois.